Zoom lens system and image pickup device having zoom lens system

ABSTRACT

To realize a zoom lens system, including, in order from an object side to an image side: a first lens unit having a negative optical power; a second lens unit having a positive optical power; a third lens unit having a negative optical power; and a fourth lens unit having a positive optical power, in which: during zooming, an interval between the first lens unit and the second lens unit at a telephoto end is smaller than that at a wide angle end, an interval between the second lens unit and the third lens unit at the telephoto end is larger than that at the wide angle end, and an interval between the third lens unit and the fourth lens unit at the telephoto end is smaller than that at the wide angle end; and specific conditions are satified.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a zoom lens system, and moreparticularly to a zoom lens system suitable for a silver halide filmcamera, an electronic recording type digital still camera, an electronicrecording type video camera, or the like.

2. Related Background Art

A so-called negative lead type zoom lens, in which a lens unit having anegative refractive power, is disposed on its front side has been usedfor a wide angle zoom lens in many cases, since it has characteristicsof: “a distance for close-up image pickup is relatively short”, “a fieldangle can be relatively easily widened”, “a back focus can be relativelyeasily lengthened”, and the, like.

On the other hand, in the negative lead type zoom lens, at a telephotoend, a first and a second lens units consist a positive group and athird and a fourth lens units consist a negative group, so that theentire optical system can be used as a so-called telephoto type.Therefore, there is a merit in that a focal length can easily belengthened even on the telephoto end.

A zoom lens is disclosed in, for example, Japanese Patent ApplicationLaid-open Nos. S57-011315, S58-095315, S59-229517, S60-055313,S60-087312, S61-062013, S61-123811, S62-063909, H02-136812, H04-235515,H04-163415 (corresponding to U.S. Pat. No. 5,132,848 B), H05-019170(corresponding to U.S. Pat. No. 5,264,965 B), H05-313065, H06-082698,H07-287168 (corresponding to U.S. Pat. No. 5,585,970 B), Japanese PatentApplication No. 2000-338397, and JP 2629904 B (specification). The zoomlens includes four lens units having negative, positive, negative, andpositive refractive powers, in order from the object side to the imageside. Zooming is performed by moving at least two lens units of thesefour lens units.

A zoom lens for single-lens reflex digital camera requires a furtherimprovement in image quality as compared with conventional zoom lensesfor silver halide film camera.

In general, when the refractive power of each of the lens units isincreased in a zoom lens, the moving amount of each of the lens unitsnecessary for obtaining a predetermined zoom ratio reduces. Therefore,it is possible to widen the field angle while the length of the entirelens system is shortened. However, when the refractive power of each ofthe lens units is merely increased, a variation in aberration withzooming becomes larger. In particular, when a wide field angle is to beobtained, it is hard to obtain preferable optical performance over theentire zoom range.

When an ultra wide field angle is set, it is hard to correctastigmatism. Therefore, there has been such a tendency that highperformance cannot be obtained or a size of the zoom lens increases.

SUMMARY OF THE INVENTION

The present invention has been made in view of the above-mentionedcircumstances in the conventional zoom lens. An object of the presentinvention is to provide a small size zoom lens system having highoptical performance over an entire zoom range with a wide field angle.

In order to solve the above-mentioned problems, an illustrated zoom lenssystem of the present invention includes a first lens unit having anegative refractive power (optical power=a reciprocal of a focallength), a second lens unit having a positive refractive power, a thirdlens unit having a negative refractive power, and a fourth lens unithaving a positive refractive power in order from the object side to theimage side. Upon zooming, at a telephoto end as compared with a wideangle end, an interval between the first lens unit and the second lensunit reduces, an interval between the second lens unit and the thirdlens unit increases, and an interval between the third lens unit and thefourth lens unit reduces. A zoom ratio is about 2.5 to 4.0. A back focusto a maximum image height and a distance between a lens plane closest tothe object side and a lens plane closest to the image side at the wideangle end are set to suitable values.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a lens sectional view showing a zoom lens at wide angle end,according to Numerical Embodiment 1;

FIG. 2 is an aberration graph of the zoom lens at wide angle end,according to Numerical Embodiment 1;

FIG. 3 is an aberration graph of the zoom lens at telephoto end,according to Numerical Embodiment 1;

FIG. 4 is a lens sectional view showing a zoom lens at wide angle end,according to Numerical Embodiment 2;

FIG. 5 is an aberration graph of the zoom lens at wide angle end,according to Numerical Embodiment 2;

FIG. 6 is an aberration graph of the zoom lens at telephoto end,according to Numerical Embodiment 2;

FIG. 7 is a lens sectional view showing a zoom lens at wide angle end,according to Numerical Embodiment 3;

FIG. 8 is an aberration graph of the zoom lens at wide angle end,according to Numerical Embodiment 3;

FIG. 9 is an aberration graph of the zoom lens at telephoto end,according to Numerical Embodiment 3;

FIG. 10 is a lens sectional view showing a zoom lens at wide angle end,according to Numerical Embodiment 4;

FIG. 11 is an aberration graph of the zoom lens at wide angle end,according to Numerical Embodiment 4;

FIG. 12 is an aberration graph of the zoom lens at telephoto end,according to Numerical Embodiment 4;

FIG. 13 is a lens sectional view showing a zoom lens at wide angle end,according to Numerical Embodiment 5;

FIG. 14 is an aberration graph of the zoom lens at wide angle end,according to Numerical Embodiment 5;

FIG. 15 is an aberration graph of the zoom lens at telephoto end,according to Numerical Embodiment 5; and

FIG. 16 is a main part schematic view showing a single-lens reflexcamera.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Hereinafter, a zoom lens system and an image pickup device having thezoom lens system according to an embodiment of the present inventionwill be described with reference to the drawings.

FIGS. 1, 4, 7, 10, and 13 are lens sectional views showing a zoom lensat a wide angle end, according to Numerical Embodiments 1 to 5(hereinafter collectively called “this embodiment”) described later,respectively. FIGS. 2 and 3 are various aberration graphs of the zoomlens at wide angle end and telephoto end according to NumericalEmbodiment 1. FIGS. 5 and 6 are various aberration graphs of the zoomlens at wide angle end and telephoto end according to NumericalEmbodiment 2. FIGS. 8 and 9 are various aberration graphs of the zoomlens at wide angle end and telephoto end according to NumericalEmbodiment 3. FIGS. 11 and 12 are various aberration graphs of the zoomlens at wide angle end and telephoto end according to NumericalEmbodiment 4. FIGS. 14 and 15 are various aberration graphs of the zoomlens at wide angle end and telephoto end, according to NumericalEmbodiment 5.

The zoom lens according to this embodiment is an image pickup lenssystem used for an image pickup device such as a single-lens reflexcamera, or the like. In the lens sectional views, the left is an objectside (front) and the right is an image side (rear).

In the lens sectional views, reference L1 denotes a first lens unithaving a negative refractive power (optical power=the reciprocal of afocal length), L2 denotes a second lens unit having a positiverefractive power, L3 denotes a third lens unit having a negativerefractive power, and L4 denotes a fourth lens unit having a positiverefractive power. Reference SP denotes an aperture stop, which isdisposed in the second lens unit L2. Reference IP denotes an image planewhere a photosensitive surface of a silver halide film or a solid-stateimage pickup element (photoelectric transducer) such as a CCD sensor ora CMOS sensor is disposed.

In the zoom lens according to this embodiment, the respective lens unitsare moved for zooming from wide angle end to telephoto end, so as toreduce an axial interval between the first lens unit L1 and the secondlens unit L2, to increase an axial interval between the second lens unitL2 and the third lens unit L3, and to reduce an axial interval betweenthe third lens unit L3 and the fourth lens unit L4. As shown by arrowsin the lens sectional views, the second lens unit L2, the third lensunit L3, and the fourth lens unit L4 are moved for magnification towardthe object side. The first lens unit L1 is moved along a locus which isconvex to the image side, thereby compensating a variation in imageplane, which is caused by the magnification. Note that the second lensunit L2 and the fourth lens unit L4 are integrally moved during zoomingwithout change in those relative positions (axial intervaltherebetween), thereby simplifying a moving mechanism.

Focusing on from an object at infinity to an object at a close point isperformed by moving the first lens unit L1 toward the object side. Thefocusing using the first lens unit L1 is preferable because the amountof operation according to an object distance is kept substantiallyconstant in a range from the wide angle end to the telephoto end,thereby simplifying the moving mechanism. In the zoom type according tothis embodiment, the second lens unit L2 is unsuitable as a focusingunit because paraxial lateral magnification becomes equal magnificationduring zooming. In the case where the third lens unit L3 or the fourthlens unit L4 is used as the focusing unit, when an interval between therespective lens units is widened to ensure the amount of movement, asize of the lens system increases. And it is not preferable because thenumber of lenses increases in order to suppress a variation inaberration during focusing.

In the zoom type according to this embodiment, a back focus (in axialdistance between a surface vertex of a final lens surface and a paraxialimage plane) becomes the shortest at wide angle end. Therefore, in orderto ensure a back focus, it is necessary to provide a refractive powerarrangement in which an image side principal point is located closer tothe image side at wide angle end. In order to locate the image sideprincipal point closer to the image side, a so-called retrofocus(inverted telephoto) type, in which a negative refractive powercomponent is located on the object side and a positive refractive powercomponent is located on the image side, may be used.

In this embodiment, the second lens unit L2, the third lens unit L3, andthe fourth lens unit L4, in which a composite refractive power ispositive, are arranged apart from the first lens unit L1 having thenegative refractive power at the wide angle end. Even in a partialsystem which is composed of the second lens unit L2, the third lens unitL3, and the fourth lens unit L4, the third lens unit L3 having thenegative refractive power is located as close as possible to the objectside in the partial system so as to locate the image side principalpoint closer to the image side. By suitably arranging the respectivelens units at the wide angle end, the zoom lens according to thisembodiment ensures a sufficient back focus in the entire system.

On the other hand, in order to shorten the lens length of the entiresystem, it is preferable that the respective lens units be arranged atthe telephoto end so as to locate the image side principal point closerto the object side. To locate the image side principal point closer tothe object side, a so-called telephoto type, in which a positiverefractive power component is located on the object side and a negativerefractive power component is located on the image side, may be used. Inthis embodiment, at the telephoto end, the first lens unit L1 and thesecond lens unit L2 are made close to each other to compose a partialsystem having a positive composite refractive power and the third lensunit L3 and the fourth lens unit L4 are made close to each other tocompose a partial system having a negative composite refractive power.By suitably arranging the respective lens units at the telephoto end asdescribed above, the entire optical length of the zoom lens according tothis embodiment at the telephoto end is shortened.

One of the features of the zoom lens disclosed in this embodiment isthat the following conditional expressions (1) and (2) aresimultaneously satisfied. Therefore, a desirable back focus is ensuredwith a state in which a suitable balance is achieved between opticalperformance and a zoom ratio in the zoom lens composed of the fourunits.2.5<ft/fw<4.0  (1)2.2<bfw/H<3.0  (2)where

fw: a focal length of the entire system at the wide angle end

ft: a focal length of the entire system at the telephoto end

bfw: a back focus at the wide angle end

H: a maximum image height

The conditional expression (1) relates to a ratio between the focallength of the entire system at the wide angle end and the focal lengthof the entire system at the telephoto end. The conditional expression(1) specifies a zoom ratio (magnification ratio) best suitable for thestructure of the zoom lens of the present invention. When the zoom ratiobecomes smaller than a lower limit value in the conditional expression(1), the zoom ratio can be realized even in the case of the zoom lenscomposed of two units. Therefore, it is useless to construct the zoomlens using the four lens units (advantage obtained from the four-unitstructure cannot be improved). On the other hand, when the zoom ratioexceeds an upper limit value in the conditional expression (1), in orderto ensure an axial interval between the first lens unit L1 and thesecond lens unit L2 at the telephoto end, it is necessary to locate thefirst lens unit L1 closer to the object side at the wide angle end.Therefore, an effective diameter of the first lens unit L1 increases andit becomes hard to correct coma aberration and distortion, with theresult that high optical performance cannot be obtained.

Note that it is more preferable to set the lower limit value in theconditional expression (1) to 2.8. The upper limit value is morepreferably set to 3.5.

The conditional expression (2) relates to a ratio between the maximumimage height and the back focus at the wide angle end, and is used tosufficiently ensure a desirable back focus.

When the ratio becomes smaller than a lower limit value in theconditional expression (2), the back focus becomes shorter for themaximum image height. Therefore, the ratio is not suitable for anoptical device that requires a predetermined back focus such as an imagepickup lens for a single-lens reflex digital camera. Further, because anexit pupil position becomes closer to an image, the ratio is notsuitable for a camera using a solid-state image pickup element thatrequires telecentricity to the image side. On the other hand, when thezoom ratio exceeds an upper limit value in the conditional expression(2), the entire lens length at the wide angle end lengthens and theeffective diameter of the first lens unit L1 increases, so that the lenssystem becomes unbalanced. In addition, it is hard to correct the comaaberration and the distortion occurring in the first lens unit L1.Although the coma aberration and the distortion can be corrected usingan aspherical lens, an increase in cost occurs.

Note that it is more preferable to set the lower limit value in theconditional expression (2) to 2.4. The upper limit value is morepreferably set to 2.8.

Another feature of the zoom lens disclosed in this embodiment is tosatisfy the following conditional expression (3) instead of theconditional expression (2), together with the conditional expression(1). Therefore, a small size zoom lens is realized in which a suitablebalance is achieved between the optical performance and the zoom ratioin the zoom lens composed of the four units.2.5<ft/fw<4.0  (1)4.1<TDw/fw<5.0  (3)where

fw: a focal length of the entire system at the wide angle end

ft: a focal length of the entire system at the telephoto end

TDw: axial distance between a lens surface closest to the object side(first lens surface) and a lens surface closest to the image side (finallens surface) at the wide angle end

The conditional expression (1) is as described above.

The conditional expression (3) relates to a ratio between a distancebetween the lens surface closest to the object side (object side surfaceof the lens closest to the object side) and the lens surface closest tothe image side (image side surface of the lens closest to the imageside) at the wide angle end and the focal length of the entire system atthe wide angle end. The ratio is mainly used to achieve a balancebetween a reduction in size and the performance.

When the ratio becomes smaller than a lower limit value in theconditional expression (3) to shorten the entire lens length at the wideangle end, the refractive power of each of the lens units becomes sostrong that it is hard to correct the respective aberrations. On theother hand, when the ratio exceeds an upper limit value in theconditional expression (3), the entire lens length lengthens and a lenseffective diameter (in particular, the effective diameter of the firstlens unit L1) increases.

Note that it is more preferable to set the lower limit value in theconditional expression (3) to 4.3. The upper limit value is morepreferably set to 4.9.

The zoom lens according to this embodiment discloses an arrangement inwhich the conditional expressions (1) to (3) are simultaneouslysatisfied.

As described above, by simultaneously satisfying the conditionalexpressions (1) and (2) or the conditional expressions (1) and (3), asmall size zoom lens system having high optical performance over theentire zoom range at a wide field angle is realized.

The zoom lens according to this embodiment satisfies the followingconditions (A) to (D). Satisfying the conditions provides the effectsdescribed above.

(A) The first lens unit L1 has a positive lens closest to the objectside. A condition−0.3<R1/R2<0.3  (4)is satisfied, where R1 represents a curvature radius of an object sidelens surface of the positive lens, and R2 represents a curvature radiusof an image side lens surface thereof.

The zoom lens according to this embodiment is a lens having a back focusrelatively longer for the maximum image height, so that the lenseffective diameter of the first lens unit L1 can easily increase. Notethat, at the wide angle end, it is hard to simultaneously correct thedistortion and the coma aberration which are caused by the negativerefractive power of the first lens unit L1. Therefore, the positive lensis located closest to the object side in the first lens unit L1 and thedistortion is actively corrected using the positive lens, therebycorrecting total distortion and the coma aberration in the first lensunit L1. The conditional expression (4) relates to a ratio between thecurvature radius of the object side lens surface of the positive lensand the curvature radius of the image side lens surface thereof, and ismainly used to achieve a balance between the distortion and a reductionin size.

When the ratio becomes smaller than a lower limit value in theconditional expression (4), it is not preferable that, in particular,astigmatism at the wide angle end increases and the lens diameterlengthens. On the other hand, when the ratio exceeds an upper limitvalue in the conditional expression (4), an effect of the distortion onthe image side lens surface of the positive lens reduces. Therefore, itis not preferable that the correction of the distortion at the wideangle end becomes insufficient. Note that it is more preferable to setthe lower limit value in the conditional expression (4) to −0.1. Theupper limit value is more preferably set to 0.15.

(B) A condition0.1<(f1/ft)²<0.5  (5)is satisfied, where f1 represents a focal length of the first lens unitL1.

The conditional expression (5) relates to a ratio between the focallength of the first lens unit L1 used as the focusing unit and the focallength of the entire system at the telephoto end, and is mainly used toreduce the size of the zoom lens.

When the ratio becomes smaller than a lower limit value in theconditional expression (5) and thus the refractive power of the firstlens unit L1 becomes too strong, the distortion occurring in the firstlens unit L1 increases. Thus, it is not preferable that it is hard tocorrect the distortion by the subsequent units. On the other hand, whenthe ratio exceeds an upper limit value in the conditional expression (5)and thus the refractive power of the first lens unit L1 becomes tooweak, the amount of operation of the first lens unit L1 increases atfocusing on a nearest object. In addition, the entire optical length atthe wide angle end lengthens to increase the lens effective diameter ofthe first lens unit L1. Note that it is more preferable to set the lowerlimit value in the conditional expression (5) to 0.19. The upper limitvalue is more preferably set to 0.4.

(C) The second lens unit L2 includes a first lens sub-unit having apositive refractive power, the aperture stop SP, and a second lenssub-unit having a positive refractive power, which are disposed in orderfrom the object side to the image side. Following conditions0.01<Lp/TD2<0.5  (6)0.2<f2/f2a<0.6  (7)are satisfied, where Lp represents an axial distance between a lenssurface of the first lens sub-unit closest to the image side and theaperture stop SP, TD2 represents a thickness of the second lens unit L2,f2 represents a focal length of the second lens unit L2, and f2 arepresents a focal length of the first lens sub-unit.

When the aperture stop SP is disposed as close as possible to the objectside, the entrance pupil is located closer to the object side.Therefore, it is preferable in view of reducing the lens diameter of thefirst lens unit L1. However, when the aperture stop SP is disposed onthe object side of the second lens unit L2, it is necessary to provide alarge distance between the first lens unit L1 and the second lens unitL2 so as not to interfere with each other in the case where the firstlens unit L1 and the second lens unit L2 are close to each other at thetelephoto end. When a desirable zoom ratio is to be ensured, the lensdiameter of the first lens unit L1 increases, with the result that thesize of the zoom lens unit cannot be reduced. On the other hand, whenthe aperture stop SP is disposed on the image side of the second lensunit L2, it is necessary to ensure a large axial interval between thesecond lens unit L2 and the third lens unit L3 particularly at the wideangle end. Therefore, the second lens unit L2 should be disposed closerto the object side. At the wide angle end, the first lens unit L1 havingthe negative refractive power is separated from the second to fourthlens units L2 to L4 having the positive composite refractive power toproduce the retrofocus type, thereby ensuring a sufficient back focus.Thus, in view of ensuring the sufficient back focus, it isdisadvantageous to locate the second lens unit L2 closer to the objectside. When the aperture stop SP is disposed on the image side of thesecond lens unit L2, a composite refractive power of the partial systemcloser to the object side than the aperture stop SP becomes weaker,thereby increasing the lens diameter of the first lens unit L1. It isnot preferable.

In this embodiment, the aperture stop SP is provided in the second lensunit L2 to realize a reduction in size of the zoom lens unit. Inaddition, the above-mentioned conditional expressions (6) and (7) aresimultaneously satisfied.

The conditional expression (6) specifies a location of the aperture stopSP in the second lens unit L2 and is used to reduce the size of the zoomlens system and to improve the performance thereof.

When Lp/TD2 becomes smaller than a lower limit value in the conditionalexpression (6) to bring the aperture stop SP close to the first lenssub-unit, the aperture stop SP and the first lens sub-unit maymechanically interfere with each other. Alternatively, because theaperture stop SP is disposed on the object side, a variation in F numberupon zooming becomes larger. On the other hand, when Lp/TD2 exceeds anupper limit value in the conditional expression (6) to dispose theaperture stop SP on the image side, the entrance pupil becomes close tothe image side. Therefore, in particular, the lens diameter of the firstlens unit L1 increases, so that it is hard to correct the distortion. Itis not preferable. Note that the upper limit in the conditionalexpression (6) is more preferably set to 0.35.

The conditional expression (7) relates to a ratio between the focallength of the second lens unit. L2 and the focal length of the firstlens sub-unit, specifies a refractive power arrangement of the secondlens unit L2, and is used for in particular, the improvement of theperformance of the zoom lens system-and the reduction in size thereof.

When the ratio becomes smaller than a lower limit value in theconditional expression (7) and thus the refractive power of the firstlens sub-unit becomes too weak, in particular, the object side principalpoint of the second lens unit L2 is located relatively close to theimage side and the second lens unit L2 approaches the first lens unit L1at the telephoto end. Therefore, it is hard to obtain a largemagnification and the entire optical length increases at the telephotoend. It is not preferable. On the other hand, when the ratio becomeslarger than an upper limit value in the conditional expression (7) andthus the refractive power of the first lens sub-unit becomes too strong,since the object side principal point of the second lens unit L2 islocated relatively close to the object side, it is hard to ensure adesirable back focus particularly at the wide angle end. When the secondlens unit L2 is located closer to the image side in order to ensure thedesirable back focus, it becomes difficult to secure the sufficientamount of movement of the third lens unit L3 relative to the second lensunit L2 upon zooming, thereby reducing an aberration correction effectof the zoom type composed of the four units. It is not preferable. Notethat it is more preferable to set the lower limit value in theconditional expression (7) to 0.3. The upper limit value is morepreferably set to 0.5.

(D) A condition,−0.75<f1/f4<−0.3  (8)is satisfied, where f1 and f4 represent the focal length of the firstlens unit L1 and a focal length of the fourth lens unit L4,respectively.

The conditional expression (8) relates to a ratio between the focallength of the first lens unit L1 and the focal length of the fourth lensunit L4, and is appropriately to set a refractive power arrangement,reduce the size of the zoom lens system while ensuring a back focus.

When the ratio becomes smaller than a lower limit value in theconditional expression (8) and thus the refractive power of the firstlens unit L1 becomes too weak, it is not preferable that the lenseffective diameter of the first lens unit L1 enlarges. On the otherhand, when the ratio becomes larger than an upper limit value in theconditional expression (8) and thus the refractive power of the firstlens unit L1 becomes too strong, it is not preferable that it is hard tocorrect the distortion, and to ensure a desirable back focusparticularly at the wide angle end. Note that it is more preferable toset the lower limit value in the conditional expression (8) to −0.70.The upper limit value is more preferably set to −0.4.

Next, the numerical embodiments will be described. In the respectivenumerical embodiments, “i” denotes an order of an optical surface fromthe object side, R1 denotes a curvature radius of an i-th opticalsurface (i-th surface), Di denotes an interval between the i-th surfaceand an (i+1)-th surface, Ni and νi denote a refractive index and an Abbenumber of a material of an i-th optical member based on a d-line,respectively. In addition, f denotes a focal length, Fno denotes an Fnumber, and ω denotes a half field angle.

With an optical axis direction being set to an x-axis, a directionperpendicular to the optical axis direction being set to a Y-axis, anaspherical shape is expressed by the following expression,

$x = {\frac{\left( {1/R} \right)h^{2}}{1 + \sqrt{\left\{ {1 - \left( {h/R} \right)^{2}} \right\}}} + {Ah}^{2} + {Bh}^{4} + {Ch}^{6} + {Dh}^{8} + {Eh}^{10} + {Fh}^{12}}$where R represents a paraxial curvature radius, A, B, C, D, E, and Frepresent aspherical coefficients. Note that “e-XX” in each of theaspherical coefficients indicates “×10^(−XX)”.

Table 1 shows correspondences between the conditional expressions andnumeral values in the respective numerical embodiments.

(Numerical Embodiment 1) f = 17.5 to 54.0 Fno = 3.56 to 5.88 2ω = 76.2to 28.5 R1 = 101.424 D1 = 3.20 N1 = 1.516330 ν1 = 64.1 R2 = −1708.614 D2= 0.12 R3 = 70.694 D3 = 1.70 N2 = 1.622992 ν2 = 58.2 R4 = 14.334 D4 =7.56 R5 = −380.821 D5 = 1.30 N3 = 1.622992 ν3 = 58.2 R6 = 24.720 D6 =0.12 R7 = 19.563 D7 = 3.10 N4 = 1.846660 ν4 = 23.9 R8 = 33.246 D8 =Variable R9 = 332.788 D9 = 1.80 N5 = 1.516330 ν5 = 64.1 R10 = Stop D10 =0.80 R11 = ∞ D11 = 2.00 R12 = 14.953 D12 = 0.75 N6 = 1.846660 ν6 = 23.9R13 = 11.113 D13 = 4.50 N7 = 1.487490 ν7 = 70.2 R14 = −69.874 D14 =Variable R15 = −34.380 D15 = 0.70 N8 = 1.625882 ν8 = 35.7 R16 = 9.803D16 = 2.75 N9 = 1.740769 ν9 = 27.8 R17 = 29.860 D17 = Variable R18 =−31.285 D18 = 1.40 N10 = 1.583060 ν10 = 30.2 *R19 = −64.486 D19 = 0.00R20 = 367.447 D20 = 4.00 N11 = 1.516330 ν11 = 64.1 R21 = −17.469 Focallength Variable Interval 17.5 29.6 54.0 D8 31.69 13.53 2.43 D14 1.453.35 6.41 D17 6.88 4.99 1.92

-   Aspherical coefficients-   Nineteenth Surface: A=0.00000e+00 B=4.73220e-05-   C=3.11152e-07 D=−1.91305e-09 E=7.87010e-12-   F=6.04742e-15

(Numerical Embodiment 2) f = 17.50 to 54.00 Fno = 3.60 to 5.88 2ω = 76.2to 28.5 R1 = 86.274 D1 = 3.40 N1 = 1.516330 ν1 = 64.1 R2 = −7682.997 D2= 0.12 R3 = 77.724 D3 = 1.70 N2 = 1.622992 ν2 = 58.2 R4 = 13.947 D4 =7.78 R5 = −260.224 D5 = 1.30 N3 = 1.622992 ν3 = 58.2 R6 = 26.430 D6 =0.12 R7 = 19.434 D7 = 3.10 N4 = 1.846660 ν4 = 23.9 R8 = 32.860 D8 =Variable R9 = 210.530 D9 = 1.80 N5 = 1.517417 ν5 = 52.4 R10 = Stop D10 =0.50 R11 = ∞ D11 = 1.50 R12 = 15.984 D12 = 0.75 N6 = 1.846660 ν6 = 23.9R13 = 11.295 D13 = 4.19 N7 = 1.487490 ν7 = 70.2 R14 = −55.465 D14 =Variable R15 = −32.125 D15 = 0.80 N8 = 1.625882 ν8 = 35.7 R16 = 10.156D16 = 2.75 N9 = 1.740769 ν9 = 27.8 R17 = 33.512 D17 = Variable R18 =−33.994 D18 = 1.40 N10 = 1.583060 ν10 = 30.2 *R19 = −54.360 D19 = 0.33R20 = −216.276 D20 = 4.10 N11 = 1.487490 ν11 = 70.2 R21 = −16.716 Focallength Variable Interval 17.50 32.05 54.00 D8 32.79 11.95 2.58 D14 1.714.76 8.19 D17 9.08 6.03 2.60

-   Aspherical coefficients-   Nineteenth Surface: A=0.00000e+00 B=4.06236e-05-   C=2.98370e-07 D=−3.36327e-09 E=2.36874e-11 F=−3.63866e-14

(Numerical Embodiment 3) f = 17.50 to 54.00 Fno = 3.60 to 5.88 2ω = 76.2to 28.5 R1 = 85.783 D1 = 3.60 N1 = 1.516330 ν1 = 64.1 R2 = 1466.008 D2 =0.12 R3 = 63.138 D3 = 1.70 N2 = 1.622992 ν2 = 58.2 R4 = 13.852 D4 = 8.60R5 = −206.618 D5 = 1.30 N3 = 1.622992 ν3 = 58.2 R6 = 26.703 D6 = 0.12 R7= 19.567 D7 = 3.10 N4 = 1.846660 ν4 = 23.9 R8 = 31.787 D8 = Variable R9= 165.349 D9 = 1.80 N5 = 1.517417 ν5 = 52.4 R10 = −32.899 D10 = 0.50 R11= Stop D11 = 2.50 R12 = 15.758 D12 = 0.70 N6 = 1.846660 ν6 = 23.9 R13 =11.294 D13 = 3.96 N7 = 1.487490 ν7 = 70.2 R14 = −64.716 D14 = VariableR15 = −32.828 D15 = 0.80 N8 = 1.625882 ν8 = 35.7 R16 = 10.201 D16 = 2.75N9 = 1.740769 ν9 = 27.8 R17 = 31.933 D17 = Variable R18 = −30.844 D18 =1.40 N10 = 1.583060 ν10 = 30.2 *R19 = −50.485 D19 = 0.21 R20 = −107.670D20 = 4.00 N11 = 1.516330 ν11 = 64.1 R21 = −16.361 Focal length VariableInterval 17.50 31.86 54.00 D8 32.55 12.08 2.63 D14 1.47 4.38 7.85 D178.49 5.58 2.11

-   Aspherical coefficients-   Nineteenth Surface: A=0.00000e+00 B=4.24038e-05-   C=3.10416e-07 D=−3.53201e-09 E=2.18966e-11-   F=2.40267e-15

(Numerical Embodiment 4) f = 17.00 to 55.00 Fno = 3.60 to 5.93 2ω = 77.8to 28.0 R1 = 126.356 D1 = 3.00 N1 = 1.572501 ν1 = 57.7 R2 = −1170.869 D2= 0.12 R3 = 56.283 D3 = 1.70 N2 = 1.772499 ν2 = 49.6 R4 = 14.558 D4 =7.56 R5 = −284.836 D5 = 1.30 N3 = 1.622992 ν3 = 58.2 R6 = 28.505 D6 =0.12 R7 = 21.111 D7 = 3.10 N4 = 1.846660 ν4 = 23.9 R8 = 40.664 D8 =Variable R9 = 117.901 D9 = 1.80 N5 = 1.517417 ν5 = 52.4 R10 = Stop D10 =1.50 R11 = ∞ D11 = 1.78 R12 = 16.224 D12 = 0.75 N6 = 1.846660 ν6 = 23.9R13 = 11.529 D13 = 4.50 N7 = 1.487490 ν7 = 70.2 R14 = −55.123 D14 =Variable R15 = −30.227 D15 = 0.80 N8 = 1.625882 ν8 = 35.7 R16 = 10.770D16 = 2.72 N9 = 1.761821 ν9 = 26.5 R17 = 28.299 D17 = Variable R18 =−31.077 D18 = 1.40 N10 = 1.688931 ν10 = 31.1 *R19 = −51.613 D19 = 0.62R20 = −125.272 D20 = 4.10 N11 = 1.487490 ν11 = 70.2 R21 = −14.837 Focallength Variable Interval 17.00 31.50 55.00 D8 36.32 13.20 2.52 D14 2.034.76 8.45 D17 7.46 4.73 1.03

-   Aspherical coefficients-   Nineteenth Surface: A=0.00000e+00 B=4.09809e-05-   C=2.36138e-07 D=−1.69273e-09 E=−1.30716e-12-   F=5.94664e-14

(Numerical Embodiment 5) f = 18.00 to 55.00 Fno = 3.59 to 5.99 2ω = 74.6to 28.0 R1 = 93.970 D1 = 3.20 N1 = 1.516330 ν1 = 64.1 R2 = −184142.483D2 = 0.12 R3 = 66.571 D3 = 1.60 N2 = 1.622992 ν2 = 58.2 R4 = 14.044 D4 =7.82 R5 = −186.656 D5 = 1.20 N3 = 1.622992 ν3 = 58.2 R6 = 27.091 D6 =0.12 R7 = 20.119 D7 = 2.90 N4 = 1.846660 ν4 = 23.9 R8 = 35.644 D8 =Variable R9 = 131.721 D9 = 1.70 N5 = 1.572501 ν5 = 57.8 R10 = Stop D10 =2.90 R11 = ∞ D11 = 2.50 R12 = 16.312 D12 = 0.80 N6 = 1.846660 ν6 = 23.9R13 = 11.532 D13 = 4.75 N7 = 1.487490 ν7 = 70.2 R14 = −57.613 D14 =Variable R15 = −33.004 D15 = 0.80 N8 = 1.620041 ν8 = 36.3 R16 = 11.180D16 = 3.20 N9 = 1.755199 ν9 = 27.5 R17 = 27.194 D17 = Variable R18 =−140.794 D18 = 1.50 N10 = 1.583060 ν10 = 30.2 R19 = −144.846 D19 = 0.12R20 = −664.185 D20 = 4.16 N11 = 1.516330 ν11 = 64.1 R21 = −19.869 Focallength Variable Interval 18.00 31.74 55.00 D8 32.31 12.13 2.05 D14 1.624.33 7.91 D17 7.72 5.02 1.44

-   Aspherical coefficients-   Nineteenth Surface: A=0.00000e+00 B=2.76705e-05-   C=1.09618e-07 D=2.25895e-11 E=−1.44537e-11-   F=1.26393e-13

TABLE 1 Conditional Numerical Numerical Numerical Numerical NumericalExpression Embodiment 1 Embodiment 2 Embodiment 3 Embodiment 4Embodiment 5 (1) ft/fw 3.09 3.09 3.09 3.24 3.06 (2) bfw/H 2.50 2.50 2.532.50 2.51 (3) 4.33 4.53 4.55 4.86 4.50 TDw/fw (4) R1/R2 −0.06 −0.01 0.06−0.11 0.00 (5) 0.22 0.22 0.21 0.22 0.23 (f1/ft)² (6) 0.08 0.06 0.05 0.150.23 Lp/TD2 (7) 0.35 0.37 0.41 0.41 0.40 f2/f2a (8) f1/f4 −0.58 −0.55−0.52 −0.57 −0.66

According to the embodiments as described above, a zoom lens system canbe realized in which the entire lens system has a small size, highoptical performance is obtained, the number of lenses is small, and astructure is simple. In particular, a zoom lens can be realized in whicha reduction in optical performance resulting from a manufacturing errorsuch as a displacement in axis of each lens is small.

Next, an embodiment in which the zoom lens system of the presentinvention is applied to an image pickup device will be described withreference to FIG. 16.

FIG. 16 is a main part schematic view showing a single-lens reflexcamera. In FIG. 16, reference numeral 10 denotes an image pickup lenshaving a zoom lens 1 according to Numerical Embodiments 1 to 5 of thepresent invention. The zoom lens 1 is held in a lens barrel 2 serving asa holding member. A camera main body 20 includes: a quick return mirror3 for reflecting a light flux from the image pickup lens 10 upward; afocal plate 4 disposed at an image forming position of the image pickuplens 10; a pentagonal roof prism 5 for converting an reverse imageformed on the focal plate 4 into an erect image; and an eyepiece lens 6for observing the erect image. Reference numeral 7 denotes a filmsurface. In image pickup, the quick return mirror 3 is removed from anoptical axis and an image is formed on the film surface 7 by the imagepickup lens 10.

The zoom lens system of the present invention is suitable for aninterchangeable lens for the single-lens reflex camera as shown in FIG.16.

This application claims priority from Japanese Patent Application No.2003-328074 filed Sep. 19, 2003, which is hereby incorporated byreference herein.

1. A zoom lens system, comprising, in order from an object side to animage side: a first lens unit having a negative optical power; a secondlens unit having a positive optical power; a third lens unit having anegative optical power; and a fourth lens unit having a positive opticalpower, wherein during zooming, an interval between the first lens unitand the second lens unit at a telephoto end is smaller than that at awide angle end, an interval between the second lens unit and the thirdlens unit at the telephoto end is larger than that at the wide angleend, and an interval between the third lens unit and the fourth lensunit at the telephoto end is smaller than that at the wide angle end,and wherein following conditions are satisfied, 2.5<ft/fw<4.0 and2.2<bfw/H<3.0, where fw and ft represent a focal length of an entiresystem at the wide angle end and that at the telephoto end,respectively, bfw represents a back focus at the wide angle end, and Hrepresents a maximum image height, wherein the first lens unit comprisesa positive lens element located closest to the object side, and whereinfollowing condition is satisfied, −0.3<R1/R2<0.3, where R1 represents acurvature radius of an object side lens surface of the positive lenselement, and R2 represents a curvature radius of an image side lenssurface of the positive lens element.
 2. A zoom lens system according toclaim 1, wherein the first lens unit moves during focusing, and whereina condition is satisfied, 0.1<(f1/ft)²<0.5, where f1 represents a focallength of the first lens unit.
 3. A zoom lens system according to claim1, wherein the second lens unit consists of a first lens sub-unit havinga positive optical power, an aperture stop, and a second lens sub-unithaving a positive optical power in order from the object side to theimage side, and wherein the following conditions are satisfied,0.01<Lp/TD2<0.5 and 0.2<f2/f2a<0.6, where Lp represents a distancebetween a lens surface of the first lens sub-unit closest to the imageside and the aperture stop, TD2 represents a thickness of the secondlens unit on an optical axis, f2 represents a focal length of the secondlens unit, and f2 a represents a focal length of the first lenssub-unit.
 4. A zoom lens system according to claim 1, wherein acondition, −0.75<f1/f4<−0.3, is satisfied, where f1 and f4 represent thefocal length of the first lens unit and a focal length of the fourthlens unit, respectively.
 5. A zoom lens system according to claim 1,wherein the zoom lens system forms an image on a photosensitive surfaceof a photoelectric transducer.
 6. An image pickup device, comprising:the zoom lens system according to claim 1; and a photoelectrictransducer for receiving an image formed by the zoom lens system.
 7. Azoom lens system, comprising, in order from an object side to an imageside: a first lens unit having a negative optical power; a second lensunit having a positive optical power; a third lens unit having anegative optical power; and a fourth lens unit having a positive opticalpower, wherein during zooming, an interval between the first lens unitand the second lens unit at a telephoto end is smaller than that at awide angle end, an interval between the second lens unit and the thirdlens unit at the telephoto end is larger than that at the wide angleend, and an interval between the third lens unit and the fourth lensunit at the telephoto end is smaller than that at the wide angle end,and wherein following conditions are satisfied, 2.5<ft/fw<4.0 and4.1<TDw/fw<5.0, where fw and ft represent a focal length of an entiresystem at the wide angle end and that at the telephoto end,respectively, and TDw represents an axial distance between a lenssurface closest to the object side of the entire system and a lenssurface closest to the image side of the entire system at the wide angleend, wherein the first lens unit comprises a positive lens elementlocated closest to the object side, and wherein a condition−0.3<R1/R2<0.3, is satisfied, where R1 represents a curvature radius ofan object side lens surface of the positive lens element and R2represents a curvature radius of an image side lens surface of thepositive lens element.
 8. A zoom lens system according to claim 7,wherein the first lens unit moves during focusing, and wherein acondition 0.1<(f1/ft)²<0.5, is satisfied, where f1 represents a focallength of the first lens unit.
 9. A zoom lens system according to claim7, wherein the second lens unit consists of a first lens sub-unit havinga positive optical power, an aperture stop, and a second lens sub-unithaving a positive optical power in order from the object side to theimage side, and wherein the following conditions are satisfied,0.01<Lp/TD2<0.5 and 0.2<f2/f2a<0.6, where Lp represents a distancebetween a lens surface of the first lens sub-unit closest to the imageside and the aperture stop, TD2 represents a thickness of the secondlens unit on an optical axis, f2 represents a focal length of the secondlens unit, and f2 a represents a focal length of the first lenssub-unit.
 10. A zoom lens system according to claim 7, wherein acondition, −0.75<f1/f4<−0.3, is satisfied, where f1 and f4 represent thefocal length of the first lens unit and a focal length of the fourthlens unit, respectively.
 11. A zoom lens system according to claim 7,wherein the zoom lens system forms an image on a photosensitive surfaceof a photoelectric transducer.
 12. An image pickup device, comprising:the zoom lens system according to claim 7; and a photoelectrictransducer for receiving an image formed by the zoom lens system.
 13. Azoom lens system, comprising, in order from an object side to an imageside: a first lens unit having a negative optical power; a second lensunit having a positive optical power; a third lens unit having anegative optical power; and a fourth lens unit having a positive opticalpower, wherein during zooming, an interval between the first lens unitand the second lens unit at a telephoto end is smaller than that at awide angle end, an interval between the second lens unit and the thirdlens unit at the telephoto end is larger than that at the wide angleend, and an interval between the third lens unit and the fourth lensunit at the telephoto end is smaller than that at the wide angle end,and wherein following conditions are satisfied, 2.5<ft/fw<4.0 and2.2<bfw/H<3.0, where fw and ft represent a focal length of an entiresystem at the wide angle end and that at the telephoto end,respectively, bfw represents a back focus at the wide angle end, and Hrepresents a maximum image height, wherein the second lens unit consistsof a first lens sub-unit having a positive optical power, an aperturestop, and a second lens sub-unit having a positive optical power inorder from the object side to the image side, and wherein followingconditions are satisfied, 0.01<Lp/TD2<0.5 and 0.2<f2/f2a<0.6, where Lprepresents a distance between a lens surface of the first lens sub-unitclosest to the image side and the aperture stop, TD2 represents athickness of the second lens unit on an optical axis, f2 represents afocal length of the second lens unit L2, and f2 a represents a focallength of the first lens sub-unit.
 14. A zoom lens system, comprising,in order from an object side to an image side: a first lens unit havinga negative optical power; a second lens unit having a positive opticalpower; a third lens unit having a negative optical power; and a fourthlens unit having a positive optical power, wherein during zooming, aninterval between the first lens unit and the second lens unit at atelephoto end is smaller than that at a wide angle end, an intervalbetween the second lens unit and the third lens unit at the telephotoend is larger than that at the wide angle end, and an interval betweenthe third lens unit and the fourth lens unit at the telephoto end issmaller than that at the wide angle end, and wherein followingconditions are satisfied, 2.5<ft/fw<4.0 and 4.1<TDw/fw<5.0, where fw andft represent a focal length of an entire system at the wide angle endand that at the telephoto end, respectively, and TDw represents an axialdistance between a lens surface closest to the object side of the entiresystem and a lens surface closest to the image side of the entire systemat the wide angle end, wherein the second lens unit consists of a firstlens sub-unit having a positive optical power, an aperture stop, and asecond lens sub-unit having a positive optical power in order from theobject side to the image side, and wherein following conditions aresatisfied, 0.01<Lp/TD2<0.5 and 0.2<f2/f2a<0.6, where Lp represents adistance between a lens surface of the first lens sub-unit closest tothe image side and the aperture stop, TD2 represents a thickness of thesecond lens unit on an optical axis, f2 represents a focal length of thesecond lens unit L2, and f2 a represents a focal length of the firstlens sub-unit.